An optical disc system is shown in FIG. 1 as a system for performing recording on and/or reproduction from an optical recording medium such as an optical disc.
A conventional optical disc system, such as that shown in FIG. 1, is arranged such that when a one-sided disc-like optical recording medium 50 is mounted therein, an optical system including a laser unit 58 and an objective lens 59 are arranged above the one-sided optical recording medium 50. The numerical aperture (referred to herein as NA) of the objective lens 59 has a value ranging from 0.50 to 0.53.
In order to drive the optical system in focusing and tracking directions with respect to the optical recording medium 50, a drive system (not shown) is provided.
The one-sided optical recording medium 50 is formed as follows. A recording layer 55 and a protective cover 56 are sequentially stacked on one surface of a light-transmitting transparent substrate 51 made of polycarbonate, for example. The light-transmitting transparent substrate 51 has a predetermined thickness t.sub.1. The thickness of a conventional substrate is typically 1.2 mm.
An operation of the conventional optical disc system is described below.
The one-sided optical recording medium 50 is placed on and driven by a rotary table (not shown). A laser beam emitted from the laser unit 58 is focused through the objective lens 59 onto the recording layer 55. A change in intensity of the focused laser beam in accordance with the data to be recorded produces pits in the recording layer 55.
In order to cope with an increase in information volume, the quantity of information signals recorded in the recording layer of an optical disc must be increased. To date, however, dimensional constraints on the optical disc and other constraints on the lens and light source prevent such increases in information volume.
One option that has been proposed is to use a two-sided optical recording medium, in which recording layers are formed respectively on both surfaces of a single optical recording medium so that information signals may be recorded on each of the surfaces.